# Deceleration distance over time

1. Jan 4, 2017

### Plebian

1. The problem statement, all variables and given/known data:
A person drives a car for 20 seconds in a straight line with an initial velocity of 20m/s. During the entire course of the journey they applied breaks causing the car to decelerate at 5m/s2. How far will it be from the starting point after the given time?

2. Relevant equations:
distance = 20 * 20 + 0.5 * -5m/s^2 * 20^2

3. The attempt at a solution:
However since I acquire a negative value, I'm convinced that my working is incorrect. What am I misunderstanding about the deceleration?

Last edited by a moderator: Jan 4, 2017
2. Jan 4, 2017

### BvU

Hello Plebian,

So what happens to the speed of the car ? As a function of time I mean.

3. Jan 4, 2017

### Plebian

I forgot to mention that I attempted the deceleration over time 20 * 20 + 0.5 * -5m/s^2 * 20 this rendered a better result that looks like it could be correct. I am on the correct track here?

4. Jan 4, 2017

### BvU

No ! it's not as if you can solve this by trial and error juggling the expressions. The dimensions don't even fit.
Now read post #2 and react to it: what happens if you slam the brakes while moving at 20 m/s and thus reduce your speed with 5 m/s per second ?

5. Jan 4, 2017

### Plebian

The velocity of 20m/s would slow down by 5m/s per second.

6. Jan 4, 2017

### BvU

Yes, that's what I wrote. So after 1 second your speed is .., after 2 ..., after 3 .... and after 4 it is zero. Would it then go negative if you keep braking ?

7. Jan 4, 2017

### Plebian

It would remain stationary at that point.

8. Jan 4, 2017

### BvU

Does the expression you use in post #1 reflect that ?

9. Jan 4, 2017

### Plebian

Not at all, which was why I initially posted this query.

10. Jan 4, 2017

### BvU

So for how long can you use that expression ?

11. Jan 4, 2017

### Plebian

Point taken.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted